1. Field of the Invention
This invention relates generally to integrated circuits which produce clock frequencies. Specifically, the present invention is a precision relaxation oscillator that produces a stable clock frequency over wide variations of ambient temperature, fabrication process and voltage. The invention is implemented on a single, monolithic integrated circuit.
2. Description of the Prior Art
The current state of the art describes RC relaxation oscillators which primarily depend on one of two schemes. In the first example as found in FIG. 1, a single comparator is coupled to a pulse generator to alternately charge and discharge a capacitor to produce a clock for a "D type" flip-flop. Several error sources are present in this design. The resistor and capacitor typically have unpredictable voltage and temperature coefficients. The charging current and comparator input slew are a function of the supply voltage which is also subject to drift. Also, the pulse generator output may vary with temperature and supply voltage. These factors lead to a clock frequency that varies over temperature.
In a second example as illustrated in FIG. 2, an RC circuit provides a common input to each of two comparators. Independent reference voltages are coupled to each of the remaining inputs of the comparators. The outputs of each of the two comparators are coupled to the inputs of a "Set-Reset type" flip-flop. The output of the flip-flop serves to alternately charge and discharge the capacitor. Although this circuit eliminates the inaccuracies of the pulse generator as discussed above in FIG. 1, other problems manifest themselves. A duty cycle error may occur since it is unlikely that the capacitor will charge and discharge at the same rate, especially over temperature variations. Also, error is induced by the difficulty of providing two reference voltages which track each other coincidently over temperature.
Therefore, a need existed to provide a relaxation oscillator which is capable of maintaining a stable clock frequency independent of temperature.